In 3GPP LTE Rel-9, transmission of a Positioning Reference Signal (PRS) in the so-called PRS subframe is enables User Equipment (UE) to detect neighbor cell sites and obtain Time-Difference Of Arrival (TDOA) measurements from the neighbor sites. The PRS is transmitted on non-control OFDM symbols of either a normal subframe or a Multimedia Broadcast over Single Frequency Network (MBSFN) subframe with ⅙ re-use in the frequency domain. In other words, the PRS is transmitted on one subcarrier out of every 6 subcarriers. Further, the shift in frequency domain is determined based on Physical Cell Identifier (PCID) as v_shift=mod(PCID, 6). Frequency reuse improves hearability or the detection Signal to Interference Ratio (SINR) of PRS. As a result, more cells can be detected and measured reliably by the UE.
The UE makes use of the Neighbor Cell List (NCL) and tests for the presence or absence of target neighbor cells by estimating the signal strength of the neighbor cell and comparing it against a threshold. Alternatively, the UE can compute a function based on the neighbor cell signal strength such as the PRS SINR of the neighbor for comparison against a threshold. If a neighbor cell satisfies a suitable threshold criterion, the UE reports to the serving cell that the neighbor was “detected”. The UE may also report the TDOA for the detected neighbor cell. If the UE reports a neighbor cell when the UE is not within the Down Link (DL) reception range of that neighbor, or equivalently, when the received signal strength of the neighbor cell is such that TDOA cannot be accurately estimated, then the report is “false”. False reports can limit the ability of the location server, especially when the false reports are persistent. In LTE Rel-9, the NCL size is 24 and the network typically includes a neighbor cell in the NCL because the UE may be in the DL reception range of that neighbor and the network does not know a priori whether or not the UE can hear that neighbor with sufficient signal strength to measure TDOA accurately or even detect that neighbor. As a result, if a UE falsely detects a neighbor repeatedly even when the UE is not within the DL reception range of the falsely detected neighbor, the location server has no way of identifying that the TDOA report for that neighbor is erroneous. The location server must then include the false report in its position coordinate computation, resulting in the degradation of positioning accuracy.
Neighbor cells with a PRS pattern that overlaps the serving cell PRS pattern (i.e., mod(PCID_neighbor, 6)=mod(PCID_serving, 6)) are strongly interfered with by the serving cell signal. The PRS signal in the frequency domain comprises a pseudo-random QPSK sequence, which is a Gold sequence mapped to a QPSK constellation, and is typically transmitted across the DL transmission bandwidth. The sequences used for different PCIDs are different allowing for a processing gain for both time and frequency domain methods for separating the PRS received from two different cells. Both the detection reliability (i.e., probability of correct detection) and TDOA estimation accuracy degrade when the serving cell received power increases relative to the target neighbor cell received power.
For neighbor cells that have a PRS pattern different from the serving cell PRS pattern (i.e., mod(PCID_neighbor, 6) not equal to mod(PCID_serving, 6)), the serving cell signal does not interfere with neighbor cell measurement when the signals are time aligned. However, when the sum of the channel delay spread and time difference of arrival exceeds the cyclic prefix (CP) of the OFDM symbol, and when the serving cell signal is comparable in strength relative to a neighbor cell, the serving cell signal “leaks” into the neighbor cell signal measurement irrespective of what method is used in receiver processing (e.g., time domain correlation with coherent/non-coherent accumulation, post-FFT frequency-domain joint signal strength estimation, etc.). This limits the detection reliability of neighbor cells and increases the false reporting rate.
Further, autonomous muting is allowed in 3GPP specification TS 36.213 v9.0.1 which means that the UE must blindly detect whether or not the PRS is present in a PRS subframe on a subframe-by-subframe basis. The UE takes a TDOA measurement only when it detects the presence of PRS. Autonomous muting enables the UE to take a TDOA measurement for a neighbor cell in subframes in which there is no serving cell PRS transmission and as a result measurements of the neighbor cell PRS are of a much higher quality from a PRS SINR perspective.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.